Remove non-existent or invalid devices from those named (#756434)

A non-existent or invalid disk device named on the command line caused
two libparted dialogs to be displayed repeatedly on every refresh.  This
was because the device was only removed from the 'device_paths' vector
when it wasn't usable [1]; not when it didn't exist or was invalid, when
the libparted ped_device_get() call failed.  Fix this.

[1] Usable device as implemented by useable_device()
    Must not have a large sector size when GParted is built with an old
    version of libparted which doesn't support large sector sizes and
    must be able to read the first sector.

Bug 756434 - GParted dumps core when passing non-existent or invalid
             device on the command line